The present invention relates to a radar system having particular, but not exclusive, application to a continuous wave radar system for predicting a range value for an intelligent cruise control in a vehicle.
Frequency Modulated Continuous Wave (FMCW) radar systems are popular because they are simple and require a lower peak power output than their pulse counterparts. U.S. Pat. No. 3,710,383 (Cherry et al.) describes an FMCW radar system used to provide a range input to a headway control system for a motor vehicle. A modulated carrier wave is transmitted towards the vehicle ahead and a reflected wave is received. The range of the vehicle ahead is determined from the frequency difference or beat frequency between the instantaneous transmitted signal and the received signal.
However, an acknowledged drawback of FMCW radar systems is that the velocity of the target (in other words the vehicle ahead) relative to the radar system affects the measured range. This is due to the fact that the velocity of the target relative to the radar antenna produces a frequency shift due to the Doppler effect which alters the frequency. The shift in frequency due to the Doppler effect alters the perceived range of the target and this phenomenon is called range-Doppler coupling.
One solution to the problem of range-Doppler coupling is to apply a modulating frequency envelope to the transmitted wave that both increases and decreases in frequency. The frequency shift on the received signal due to the Doppler effect will be positive for an approaching target and negative for a receding target regardless of the instantaneous direction of the radar frequency sweep, whereas the frequency shift due to the range of the target will alter in polarity with the direction of frequency sweep. The frequency shifts due to target range and the Doppler effect respectively can thus be distinguished. However, the use of a bi-directional frequency sweep in this manner increases the possibility of confusion between multiple targets.
An alternative solution is to measure the distance using an FMCW radar, derive the target velocity by differentiation of the distance signal (or beat frequency), and calculate the error due to range-Doppler coupling from the target velocity. This is difficult to achieve in practice, however, because small difference signals have to be derived from signals with large amplitudes and this is especially difficult to achieve successfully in a noisy environment such as a motor vehicle.
FMCW radar systems are frequently employed in control systems, for example the vehicle headway control system mentioned previously, which require an element of prediction. In a headway control system, allowance must be made for the delay in a vehicle's response to alterations in the amount of throttle applied to the engine.
FIG. 1 of the accompanying drawings shows a block schematic diagram of a prior art vehicle headway control system 10. A desired speed value DS is fed to a look-up table 12 to provide a desired headway DH signal (based on sensible stopping distances) to a control unit (CNTL) 14. The CNTL 14 is provided with a further three signals, namely a headway measurement HW, a relative velocity measurement RV and an estimated acceleration EA. The CNTL provides a control signal TC for controlling the rate of change to be applied to the vehicle throttle (not shown). The signal TC is also fed to an integrator 16 which includes a delay of a duration dependent upon the dynamics of the vehicle and which provides, in response to signal TC, the estimated acceleration signal EA to the CNTL 14. The headway signal HW is typically provided by a FMCW radar (not shown) and relative velocity RV derived by differentiating the headway signal. For the purposes of description, however the estimated acceleration signal EA is shown as being fed to another integrator 18. The integrator 18 provides an output signal SV representative of the velocity of the system to a subtracter 20 also shown for the purposes of description. A further input to the subtracter 20 is shown to be provided by the target velocity TV. The relative velocity RV between the headway control system and the vehicle ahead appears at the output of the subtracter 20 and is fed to the CNTL 14 and to a further integrator 22 shown for the purposes of description. The integrator 22 provides a value of the headway HW between the system and the vehicle ahead to the CNTL 14. The CNTL 14 is operable to adjust the rate of acceleration of the vehicle to equate the actual headway HW with the desired headway DH while taking estimated acceleration and relative velocity into consideration. The CNTL 14 may also be arranged to limit vehicle speed as measured, for example, by a radar speedometer to the value of desired speed DS.
A headway control system such as that described above requires a substantial amount of circuitry to provide the necessary prediction and careful setting-up to ensure stability.